Growth properties and composition of cytoskeletal and cytocontractile proteins in aortic cells isolated and cultured from normal and atherosclerotic rabbits

Development of atherosclerotic plaques

Atherosclerosis is a multifactorial disease. A unified model for the lesion development reveals many connections between the response-to-injury hypothesis and the lipid hypothesis. Various cellular uptake mechanisms for native and modified lipoproteins are discussed with respect to foam cell formation and lesion development. The role of arterial smooth muscle cells in atherogenesis is emphasized. Preliminary results of biochemical analyses from the coronary arteries of accidentally died Finnish children and adults reveal that the accumulation of plasma-derived cholesteryl esters in young children began early in childhood, long before the vessels became macroscopically affected and changes in arterial glycosaminoglycans correlated significantly with the accumulation of cholesteryl esters.

Proteomic approach to identify changes in protein expression modified by 17β-oestradiol in bovine vascular smooth muscle cells

The aim of the present study was to use proteomics to analyse modifications in the level of expression of different proteins in BVSMCs (bovine vascular smooth muscle cells) incubated in the absence and presence of 17β-oestradiol. By using two-dimensional electrophoresis with a pH range of 4–7, we identified several areas on the gels in which the level of expression of proteins were different between control BVSMCs and cells incubated for 24 h with 17β-oestradiol. Changes in several isoforms of α-enolase, HSP60 (heat-shock protein 60), vimentin and PDI (protein disulphide-isomerase) were observed in BVSMCs. The expression of α-enolase isoform 1 was enhanced after 17β-oestradiol treatment. The expression of HSP60 isoform 3, vimentin isoforms 2 and 3 and caldesmon was reduced by 17β-oestradiol. Finally, the expression of PDI isoforms was reduced by 17β-oestradiol. In summary, 17β-oestradiol modified the expression of isoforms of proteins associated with smooth muscle cell proliferation (α-enolase, vimentin and HSP-60), cell contraction (vimentin and caldesmon) and cell redox modulation (PDI). These findings confirm that 17β-oestradiol may modulate a wide range of signalling pathways in vascular smooth muscle cells.

Phenotype modulation in primary cultures of aortic smooth muscle cells from streptozotocin-diabetic rats

Diabetes mellitus is a major risk factor for atherosclerosis. In atherosclerotic lesions, arterial smooth muscle cells (SMC) change from a contractile to a synthetic phenotype characterized by active proliferation. A similar phenotype modulation occurs in vitro when isolated arterial SMC are grown in culture and is characterized by both changes in cell morphology and a typical switch in actin isoform expression. In this study, we examined the influence of streptozotocin (STZ)-induced diabetes on the differentiation state and the phenotype modulation of cultured rat aortic SMC. We used transmission electron microscopy to study the fine structure of STZ-diabetic and non-diabetic SMC in primary culture and immunological methods for the determination of the proportions of alpha-smooth muscle actin (alpha-SM) and nonmuscle beta-actin (beta-NM) isoforms. Cultured STZ-diabetic SMC exhibited a large cytoplasmic volume, rich in rough endoplasmic reticulum, when compared with cultured non-diabetic SMC. alpha-SM, organized in stress fibers, was less homogeneously and abundantly distributed and by contrast, beta-NM was more abundant in STZ-diabetic than in non-diabetic SMC. Cytofluorimetric analyses demonstrated that the alpha-SM content was reduced in freshly STZ-diabetic SMC. Furthermore, during logarithmic growth of cultured SMC, the decrease of alpha-SM was more important in STZ-diabetic than in non-diabetic SMC. Immunoblotting of actin isoforms confirmed that expression of beta-NM was more important in STZ-diabetic than in non-diabetic SMC even in freshly isolated cells. The results suggest that SMC from STZ-diabetic rats express a more dedifferentiated state and undergo a more rapid phenotypic modulation in primary cultures than SMC from non-diabetic rats. Therefore, diabetes could induce changes in the phenotype of arterial SMC which might be associated with the onset or progression of the atherogenic process.

Intermediate filaments and ATPase activity in the vascular wall of vertebrates

The vascular wall of aorta and vena cava was examined for adenosine triphosphatase (ATPase) activity and cytoskeletal intermediate filaments (IF) in different representatives of vertebrates. Enzyme activity was studied by the modified method of Padykula and Herman. A streptavididin-biotin immunohistochemical method was applied to reveal desmin (D) and vimentin (V) IF. Endothelial cells of all vessels were V-positive and D-negative and exhibit high ATPase activity. Vascular smooth muscle cells (SMC) in lower vertebrates (pisces and amphibia) were also V-positive and D-negative, but showed low ATPase activity. SMC were D-positive and V-negative and possessed high enzyme activity in aves and mammals, similar to that of the endothelium. In cow vascular wall D-reactivity and high ATPase activity were mostly expressed in bundles of mosaically arranged thick SMC fibres of the outer aortic media as well as in the longitudinal fibres in the inferior vena cava. In higher vertebrates SMC of vasa vasorum were both V- and D-positive and showed high enzyme activity. The results demonstrate that D-immunoreactivity is mostly expressed in SMC of layers of high functional activity, which correlates with the intense ATPase reaction in these cells.

Identification of the phenotypic modulation of rabbit arterial smooth muscle cells in primary culture by flow cytometry

In atherosclerotic lesions, smooth muscle cells (SMC) change from a contractile to a synthetic phenotype. The in vivo and in vitro phenotypic transformations of SMC have been confirmed by transmission electron microscopy (TEM), but the relationship between this change and the cell cycle is still unknown. We demonstrated the structural modulation of rabbit arterial SMC in primary culture by TEM and immunocytochemistry and simultaneously studied changes in two-dimensional histograms of the relative DNA and RNA contents by flow cytometry. During the first day of primary culture, the cells exhibited the contractile phenotype and were composed of a population in the G0 phase characterized by low contents of DNA and RNA. On the second day of culture, some of the cells (18.2%) had started but not completed the transition into the synthetic phenotype and a cell population in the G1A phase with an RNA content above the G0 level appeared in almost the same proportion. This cell population could be categorized as an "intermediate" type. Moreover, after 3 days when about three-quarters of the cells had undergone structural transition, the same proportion of cells had entered into the cycling phase, while some cells still remained in the G0 and G1A phases. Thus, cell cycle analysis by flow cytometry corresponded well with the observations obtained by TEM and immunocytochemistry. These results show that flow cytometry can rapidly and relatively conveniently monitor the process of phenotypic modulation in SMC and is a useful method for the analysis of such transitions.

Immunocytochemistry of intermediate filaments in cultured arterial smooth muscle cells: differences in desmin and vimentin expression related to cell of origin and/or plating time

The objective of this study was to determine whether intermediate filament expression, including desmin and vimentin, in cultured smooth muscle cells (SMCs) is related to cyto-differentiation or proliferation. Using antibodies to desmin and vimentin, we studied by immunoperoxidase technique the distribution of these proteins in subcultured SMCs derived from porcine aorta and coronary artery. In addition, the proliferative potentiality of the cells was estimated by the incorporation of [3H]thymidine into DNA. The frequency of desmin-positive cells in coronary arterial SMCs of 3 and 6 population doubling levels was significantly higher as compared to findings with the aortic SMCs and depended on the plating time. No difference was evident at the 12 population doubling level. In contrast, vimentin was present in the majority of both aortic and coronary arterial SMCs. With regard to the localization of vimentin, two cell types were observed, one had reaction products to vimentin in both perinuclear and cell-peripheral areas (type-I cell), the other only in the cell-peripheral region (type-II cell). The relative proportion of the type-I and -II cells varied with the period of culture. Most of the SMCs showed the type-I cell on the first day and the number of type-II cells was increased on the sixth day. Quiescent SMCs in serum-free media had the same percentage of desmin-positive cells and frequency distribution of type-I and -II cells as did the proliferating SMCs incubated in media containing 5% serum. These results suggest that intermediate filament expression, including desmin and vimentin in cultured SMCs, is related to cell origin and/or plating time, but not to the proliferating activity, per se.

Changes in the composition of cytoskeletal and cytocontractile proteins of rat aortic smooth muscle cells during aging

Cytoskeletal proteins are used as differentiation markers of vascular smooth muscle cells (SMC). To study possible changes in SMC phenotype during aging, cytoskeletal and cytocontractile proteins were quantified in the aortic intima-medias of 4-, 12-, 30-, and 36-month-old rats by one- and two-dimensional gel electrophoresis. The percentages of myosin and desmin in total protein decreased with age, while those of actin and vimentin remained unchanged. Immunohistochemical comparison of the aortas from 4- and 30-month-old rats showed that the reduction of desmin reflected a selective disappearance of desmin in some cells. There was an age-related increase in the proportion of beta-actin at the expense of the alpha-isoform. Our results suggest an age-dependent modulation of the phenotype of vascular SMC towards the synthetic state, which is opposite to that observed during developmental differentiation.

Collagen synthesis by cultured rabbit aortic smooth-muscle cells. Alteration with phenotype

Enzymically isolated rabbit aortic smooth-muscle cells (SMC) in the first few days of primary culture express a 'contractile phenotype', but with time these cells modulate to a 'synthetic phenotype'. Synthetic-state SMC are able to proliferate, and, provided that they undergo fewer than 5 cumulative population doublings, return to the contractile phenotype after reaching confluency [Campbell, Kocher, Skalli, Gabbiani & Campbell (1989) Arteriosclerosis 9, 633-643]. The present study has determined the synthesis of collagen, at the protein and mRNA levels, by cultured SMC as they undergo a change in phenotypic state. The results show that, upon modulating to the synthetic phenotype, SMC synthesized 25-30 times more collagen than did contractile cells. At the same time, non-collagen-protein synthesis increased only 5-6-fold, indicating a specific stimulation of collagen synthesis. Steady-state mRNA levels are also elevated, with alpha 2(I) and alpha 1(III) mRNA levels 30 times and 20 times higher respectively, probably reflecting increased transcriptional activity. Phenotypic modulation was also associated with an alteration in the relative proportions of type I and III collagens synthesized, contractile SMC synthesizing 78.1 +/- 3.6% (mean +/- S.D.) type I collagen and 17.5 +/- 4.7% type III collagen, and synthetic cells synthesizing 90.3 +/- 2.0% type I collagen and 5.8% +/- 1.8% type III collagen. Enrichment of type I collagen was similarly noted at the mRNA level. On return to the contractile state, at confluency, collagen production and the percentage of type I collagen decreased. This further illustrates the close association between the phenotypic state of SMC and their collagen-biosynthetic phenotype.

Lipoprotein uptake in primary cell cultures of rabbit atherosclerotic lesions. A fluorescence microscopic and flow cytometric study

To characterize the lipoprotein metabolism of lipid-filled cells of atherosclerotic lesions, uptake of 3,3'-dioctadecylindocarbocyanine (DiI)-labelled low density lipoprotein (LDL), acetylated LDL (Ac-LDL) and beta-very low density lipoprotein (beta-VLDL) was studied by fluorescence microscopy and flow cytometry in primary cultures of enzymatically dispersed aortic cells from cholesterol-fed rabbits. Most of the foam cells were identified as macrophages on the basis of Fc-receptors and high activities of nonspecific esterase and acid lipase, although cholesteryl ester (CE) inclusions were found by filipin staining also in smooth muscle cells (SMCs). During the culture only SMCs proliferated and were confluent in about 1 week. After incubation with DiI-Ac-LDL most macrophage foam cells were brightly fluorescent, but also many SMCs accumulated fluorescence. In SMCs, an excess of LDL inhibited the uptake of DiI-beta-VLDL and DiI-LDL, indicating that these lipoproteins were taken up by the apoB,E receptor; the activity of this receptor was low 2 days after cell isolation but increased considerably during SMC proliferation. DiI-beta-VLDL was not taken up by the macrophage foam cells until after 7 days' culture, when their CE content had decreased, reflecting a feed-back regulation of these receptors as well. Our results indicate that, in primary cultures of enzyme-dispersed cells from rabbit atherosclerotic lesions, most of the foam cells have lipoprotein receptors resembling those described in macrophages and that also many SMCs accumulate Ac-LDL.

Phenotype modulation in primary cultures of arterial smooth-muscle cells: reorganization of the cytoskeleton and activation of synthetic activities

During primary culture, arterial smooth-muscle cells (SMCs) undergo transition from a contractile to a synthetic phenotype. As a consequence, they lose the ability to contract and, instead, acquire the ability to synthesize DNA, divide and produce extracellular-matrix components. In the present study, we used cytochemical and electron-microscopic methods to study the organization of the cytoskeleton in primary cultures of adult rat and human arterial SMCs. Freshly isolated cells were all in contractile phenotype and stained intensely with NBD-phallacidin, a fluorescent marker for F-actin. Diffuse, positive staining was also obtained using indirect-immunofluorescence microscopy with antibodies against tubulin and vimentin, which are subunit proteins of microtubules and intermediate filaments, respectively. Fine structurally, the cytoplasm of these cells was mainly filled with microfilament bundles coalescing in dense bodies. After a few hours in culture, the SMCs attached to the substrate and started to extend processes in various directions. These stained with antibodies to tubulin and vimentin, but not with NBD-phallacidin. Within 1-3 days of culture, the cells spread out on the substrate and developed a system of actin-containing stress fibre bundles spanning their entire length, as well as a radiating system of microtubules and vimentin filaments, originating in the juxtanuclear region. Fine structurally, these changes corresponded to a marked decrease in the number of microfilaments, an increase in the number of microtubules and intermediate filaments, and the formation of an extensive rough endoplasmic reticulum and a large Golgi complex. The morphological transformation of the cells was accompanied by the coordinated activation of DNA, RNA and protein synthesis.